As described by S. E. Miller in his article entitled "Coupled Wave Theory," published in the May 1954 issue of the Bell System Technical Journal, pp. 661-719, the power that is coupled between the waveguides of a directional coupler varies as a function of the length of the coupling interval, the coupling coefficient, and the propagation constants of the two waveguides. At microwave frequencies and below, it is relatively easy to design and construct a directional coupler having any prescribed power division ratio. At optical frequencies, however, the fabrication of a directional coupler having a particular set of operating characteristics is much more difficult. Accordingly, various means have been devised to compensate, for example, for inadvertent differences in propagation constants, and to adjust the effective length of the coupler so as to obtain the desired power division. (See, for example, U.S. Pat. No. 4,012,113.) While these various techniques are effective, they tend to complicate coupler structures.
The object of the present invention is to simplify the structure of directional couplers.